The idea of function-retaining artificial replacements for intervertebral discs is younger than that for replacements of artificial joints of extremities, but in the meantime about 50 years old [Büttner-Janz, Hochschuler, McAfee (Eds.): The Artificial Disc. Springer Verlag, Berlin, Heidelberg, New York 2003]. It results from biomechanical considerations, unsatisfactory results of fusion surgeries, disorders adjacent to fusion segments and from the development of new materials with greater longevity.
The publications and other materials, including patents, used herein to illustrate the invention and, in particular, to provide additional details respecting the practice are incorporated herein by reference.
By means of function-retaining disc implants it is possible to avoid fusion surgery, i.e. to maintain, or to restore the mobility within the intervertebral disc space. In an in-vitro experiment it is also possible to achieve a normalization of the biomechanical properties of the motion segment to a large extent through the implantation of an artificial intervertebral disc after a nucleotomy.
Implants for the replacement of the whole intervertebral disc differ from those for the replacement of the nucleus pulposus. Accordingly, implants for the total replacement of the intervertebral disc are voluminous; they are implanted via a ventral approach. An implantation of a prosthesis for total replacement of the intervertebral disc immediately after a standard nucleotomy can therefore not be carried out.
The indication for a function-retaining intervertebral disc replacement as an alternative to the surgical fusion includes, besides the painful discopathy, also pre-operated patients with a so-called post discectomy syndrome, patients with a recurrent herniated intervertebral disc within the same segment and patients having a pathology within the neighbouring intervertebral disc as a consequence fusion surgery.
Presently, a total of more than 10 different prostheses are clinically used for the total replacement of intervertebral discs. For the lumbar spine the CHARITÉ Artificial Disc, the PRODISC, the MAVERICK, the FLEXICORE and the MOBIDISC (Overview in Clinical Reports, PJB Publications Ltd., June 2004) are particularly well known, and for the cervical spine the BRYAN prosthesis, the PRESTIGE LP prosthesis, the PRODISC-C and the PCM prosthesis, which will be described below.
The PRODISC prosthesis for the lumbar spine is being implanted since 1999, following its further development to the PRODISC II. Although with respect to its components a three-part intervertebral disc prosthesis, it is functionally a two-part prosthesis with its sliding partners made of metal and polyethylene. Implantations of the PRODISC are carried out in the lumbar spine and with an adapted model of the prosthesis, the PRODISC-C, also in the cervical spine. Different sizes, heights (achieved by the polyethylene core) and angles of lordosis (achieved by the metal endplates) are available. Bending forward and backward as well as to the right and left is possible to the same extent of motion; the axial rotation is not limited in the construction.
The same applies to both two-part prostheses for the cervical spine, the PCM prosthesis with its sliding partners metal and polyethylene and the PRESTIGE LP prosthesis with its sliding partners metal-metal. As special feature of the construction of the PRESTIGE LP prosthesis it has the possibility for an anterior-posterior translation, due to the horizontal ventrally prolonged concavity, which, in a frontal section, has the same radius as the convexity.
The Maverick MAVERICK and the FLEXICORE for the lumbar spine are functionally two-part prostheses with spherical convex-concave sliding partners, both with sliding partners made of metal. In contrast, the MOBIDISC is functionally a three-part prosthesis with sliding partners of metal-polyethylene and two articulation surfaces. One area is a segment of a sphere, as it is in the three afore mentioned prostheses, with a convex and a concave surface of the articulating partners each of the same radius, the other area of the MOBIDISC being plane. Although a limitation of the axial rotation is planned within the plane section, it is not limited within the convex-concave area of articulation. In contrast the FLEXICORE has a small stopping area within the spherical sliding surfaces limiting the rotation movement.
The BRYAN prosthesis is clinically used as a compact prosthesis for total replacement of intervertebral discs of the cervical spine. It is attached to the vertebral bodies by convex titanium plates with a porous surface and achieves its biomechanical properties by virtue of a polyurethane nucleus.
The longest experience exists with the CHARITÉ prosthesis, which is the subject matter of the DE 35 29 761 C2 and the U.S. Pat. No. 5,401,269. This prosthesis was developed in 1982 by Dr. Schellnack and Dr. Buttner-Janz at the Charité in Berlin and was later on named SB CHARITÉ prosthesis. In 1984 the first surgery took place. The intervertebral disc prosthesis was further developed and since 1987 the current type of this prosthesis, model III, is being implanted; in the meantime over 10000 times worldwide (DE 35 29 761 C2, U.S. Pat. No. 5,401,269). The prosthesis is functionally three-parted with the sliding partners being metal and polyethylene with two identical spherical sliding surfaces. On the one hand it has a transversally mobile polyethylene core and on the other hand the accordingly adapted concave cups within the two metal endplates. For the adaptation to the intervertebral space, the CHARITÉ prosthesis provides different sizes of the metal plates and different heights of the size adapted sliding cores as well as angled prosthetic endplates, which when implanted vice versa in sagittal direction can also be used as replacement for the vertebral body. The primary fixation of the CHARITÉ prosthesis is achieved by six teeth, which are located in groups of three slightly towards the middle next to the frontal and rear edge of each prosthetic plate.
The other prosthesis have other primary fixations on their surfaces directed towards the intervertebral bodies, e.g. a sagitally running keel, a structured surface, a convex shape with for instance crosswise running grooves and combinations thereof, also with differently located teeth. Furthermore screw fixations can be used, either from ventral or from within the intervertebral space into the intervertebral body.
To assure a long-term fixation of the prosthetic endplates to the intervertebral bodies and to thus generate a firm connection with the bone, a surface was created in analogy to cement-free hip and knee prostheses, which combines chrome-cobalt, titanium and calcium phosphate in such a way that it is possible for bone to grow directly onto the endplates. This direct connection between prosthesis and bone, without the development of connective tissue, makes a long-term fixation of the artificial intervertebral disc possible and reduces the danger of loosening or displacements of the prosthesis and material breakage.
One primary objective of function retaining intervertebral disc replacements is to closely adapt the motions of the prosthesis to the ones of a healthy intervertebral disc. Directly connected to this is the motion and stress for the facet joints, which following inappropriate biomechanical stress have their own potential for disorders. There can be abrasion of the facet joints (arthritis, spondylarthritis), in the full blown picture, with the formation of osteophytes. As result of these osteophytes and also by a pathologic course of motion of the intervertebral disc alone, the irritation of neural structures is possible.
A healthy intervertebral disc is, in its interactions with other elements of the motion segment, composed in such a way that it allows only motions to a certain extent. For example, within the intervertebral disc, motions to the front and back are combined with rotary motions, and side motions are also combined with other motions. The motion amplitudes of a healthy intervertebral disc are very different, with respect to the extension (reclination) and flexion (bending forward) as well as to the lateral bending (right and left) and rotary motion. Although of common basic characteristics, there are differences between the motion amplitudes of the lumbar and cervical spine.
During motion of the intervertebral disc the centre of rotation changes, i.e. the motion of the intervertebral disc does not take place around a fixed center. Due to a simultaneous translation movement of the adjacent vertebrae, the center changes its position constantly (inconstant center of rotation). The prosthesis according to DE 35 29 761 C2 shows a construction which differs in comparison to other available types of prostheses which are build like a ball and socket joint, as a result of which they move around a defined localized centre of rotation. By virtue of the three-part assembly of the prosthesis according to DE 35 29 761 C2, with two metallic endplates and the interpositioned freely mobile polyethylene sliding core, the course of motion of a healthy intervertebral disc of the human spine is mimicked as far as possible, however without the exact motion amplitudes in the specific motion directions.
A further important feature of the healthy lumbar intervertebral disc is its trapezium shape, which is primarily responsible for the lordosis of the lumbar and cervical spine. The vertebral bodies themselves contribute only to a minor extent to the lordosis. During prosthetic replacement of intervertebral discs the lordosis should be maintained or reconstructed. The Charité disc prosthesis provides four differently angled endplates, which moreover can be combined with each other. However during surgery there is more surgical effort and the risk to damage the vertebral endplates with the resulting danger of subsidence of the prosthesis into the vertebral bodies, if the prosthesis has to be removed completely, because a good adjustment of lordosis and an optimal load of the center of the polyethylene core were not achieved.
To avoid sliding or a slip-out of the middle sliding partner from the endplates, the DE 35 29 761 C2 discloses a sliding core with a two-sided partly spherical surface (lenticular), with a plane leading edge and at the exterior with a ring bulge, which will lock between the form-adapted endplates during extreme motion. The DE 102 42 329 A1 discloses a similar intervertebral disc prosthesis which has a groove around the contact surfaces, in which an elastic ring is embedded that is in contact with the opposite contact area for a better course.
The EP 0 560 141 B1 describes a three-part intervertebral disc prosthesis, which also consists of two endplates and an interpositioned prosthetic core. The intervertebral disc prosthesis, described in this document, provides a resistance during rotation of its endplates in opposing directions around a vertical rotary axis without a contact between the prosthetic endplates. This is achieved by a soft limitation of the endplates during rotation onto the prosthesis core caused by the weight, which acts on the plates as a result of the biomechanical load transfer within the spine, because the corresponding radii of curvature differ in a median-sagittal and frontal transection.
The above mentioned models are permanently anchored in the intervertebral spaces as implants. Especially due to a load transfer over too small surface areas, a migration of the endplates into the vertebral bodies and thus a dislocation of the complete implant is possible in middle to long-term, resulting in artificial stress for the vertebral bodies and the adjacent nerves and in the end for the total motion segment, and leading to new complaints of the patients. The long-term stability of the polyethylene and the restricted mobility of the intervertebral disc prosthesis due to an inappropriate load on the polyethylene within the intervertebral space have to be discussed. Insufficiently adapted ranges of motion and adverse biomechanical stress in the motion segment can possibly lead to persistence of the complaints or later on to new complaints of the patients.
The U.S. Pat. No. 6,706,068 B2 on the other hand, describes an intervertebral disc prosthesis comprising an upper and lower part, in which the parts are built correspondently towards each other. No intermediate part as middle sliding partner exists. Different designs are realized for the interdigitating and articulating partners, resulting in a two-part prosthesis. The design is however limited to structures having either edges or corners so that this way both parts of the prosthesis articulate with each other; in this case it is not possible to speak of sliding partners. Furthermore two sliding partners are described having one convex part towards the interior of the prosthesis and the other sliding partner is correspondingly shaped concavely. This kind of prosthesis, however, allows restricted movements of the artificial intervertebral disc only. The concave protuberance corresponds to a part of a ball with the according radius. The U.S. Pat. No. 6,706,068 B2 further shows a two-part disc prosthesis having convex and concave partial areas on each sliding partner corresponding to concave and convex partial areas of the other sliding partner. According to the disclosure of the U.S. Pat. No. 6,706,068 B2 several fixed points of rotation are generated.
The U.S. Pat. No. 5,258,031 discloses a two-part disc prosthesis, in which the two endplates articulate with each other by a ball and socket joint. The joint is located centrally in the frontal section. In a lateral view, the small area of articulation is positioned outside the middle. The articulation areas are spherical in a sagittal view and plane in a frontal view, at the ends small and partly spherical parts are followed by plane skewed ones; these parts have no contact when the other parts of the joint are in contact. Bending to one side with a prosthesis according to the U.S. Pat. No. 5,258,031 is achieved by use of the partly spherical edge of the articulation areas. Whether the lateral inner parts of the endplates come into contact with each other cannot be clearly discerned from the U.S. Pat. No. 5,258,031. At least during one lateral motion, the laterally outwardly opened areas in the bilateral part of the articulation areas do not come into contact. Therefore during lateral bending of the endplates according to the U.S. Pat. No. 5,258,031 the pressure is partly on the spherical edges of the articulation areas only. Because of the pressure distribution only onto points or small areas during side bending, the outer parts of the convex/concave parts are exposed to greater abrasion. The edges of the prosthesis also do not have contact over a large area during the different movements. If the prosthesis according to the U.S. Pat. No. 5,258,031 provides rotation around a vertical axis there is only a bilateral punctiform contact area between the upper and lower endplates.
Thus, there is a need for an intervertebral disc prosthesis for the total replacement of intervertebral discs, with which the extent of the movement can be specifically adapted to the anatomy and biomechanics of the lumbar and cervical spine. This is achieved by the design of the edges of the prosthesis by being such that a physiological motion and at the same time as large as possible a contact region of the sliding partners is provided.
This need is addressed by the present invention. The invention comprises two different types of an intervertebral disc prosthesis, namely a functionally two-part and a functionally three-part prosthesis.